This invention is concerned with improvements in moulding articles from pastes or slurries comprised predominantly of particulate inorganic materials with or without binders.
The invention is particularly although not exclusively concerned with the manufacture of high strength light weight masonry or ceramic articles and apparatus and methods for producing such apparatus.
Particulate inorganic compositions containing a cementitious binder or inorganic materials which react chemically to form a binding medium for the inorganic compositions are well known and include silica sands, portland cement, gypsum, lime, fly ash or other pozzolans, aluminosilicates, clays, and the like. These materials, which may or may not include polymeric organic binders for added strength or other organic materials such as cellulosic or polymeric fibres, are often used in the manufacture of building materials such as masonry blocks, wall panels and the like.
Other particulate inorganic compositions, typically clays and other ceramic materials can be moulded into shapes and fired to form high strength articles such as building bricks, pavers and the like.
In the case of masonry or ceramic articles, particulate materials are often packed into a mould as an aqueous slurry or flowable paste and then are dewatered to form a stiff “green” article able to be handled after removal from a mould for subsequent curing or firing.
It is well understood that mixtures of particulate materials and water or other liquid can be dried or dewatered using vacuum applied behind a filter membrane placed between the material to be so dried and the vacuum.
The process is normally used to extract the liquid from the solid so that either the solid or liquid is recovered for further processing or sale.
In the manufacture of masonry articles with a cementitious composition, a compromise must be made in determining the water content of such a composition.
To maximize the strength of the masonry article, only sufficient water to hydrate the cement powder or plasticize the mix should be added to the composition. Invariably this results in a relatively dry mixture which is difficult to mix thoroughly in a conventional mixer and this dry mixture is difficult to handle in a mould due to its stiff non-flowable nature.
While mixing and handling of cementitious compositions is assisted by adding an excess of water to produce a flowable paste or slurry, the strength of the resultant cured product may be substantially lessened by shrink cracking and the like.
Dewatering of concrete can remove water from concrete after placing in a mould so as to lower the water cement ratio which will normally have the effect of increasing the rate of set of the concrete and/or improving properties such as strength or water-tightness. Dewatering of concrete is carried out using either applied pressure directly, or indirectly by applying vacuum to the mass of concrete via simple perforated screen.
Typically, a concrete building panel can be dewatered in a horizontal mould having a perforated screen floor and a perforated screen mould top. A filter medium of paper or fabric is located between the upper and lower screens and the upper and lower faces of the panel.
After filling the mould with a flowable concrete slurry, a mechanical or hydraulic force is applied to the upper screen to force excess water out of the concrete mass to produce a moulded article stiff enough to remove from the mould.
Vacuum dewatering utilizes a similar mould structure encased in a flexible membrane enclosure. When the interior of the membrane enclosure is evacuated, atmospheric pressure applied to the upper screen surface via the membrane effects dewatering in a manner similar to the mechanical or hydraulic process described above.
Dewatering of clay materials can be very difficult due to their viscosity.
Dewatering systems operating in this manner are difficult to operate and can normally only produce simple flat or flat-sided components. As the water is drawn out of the article being dewatered the volume of the article will decrease. It is not possible to mould a three dimensional shape with accurate dimensions without using a mould that is designed to “shrink” while the dewatering or compacting process is underway. Usually however, in the case of moulded concrete articles, a slurry is introduced into an open topped mould and the mass is allowed to shrink within the mould, initially with the height of the article subsiding and subsequently the upright walls of the moulded article shrink away from the mould surface. Dewatering in this manner is very slow and would not normally be used in a high capacity production process.
It is an aim of this invention to provide an improved method and apparatus to rapidly dewater pastes or slurries of particulate inorganic materials in the production of moulded articles in such a way that the shape and/or volume of the article remains substantially the same throughout the dewatering process.